Lance for immersion in a pyrometallurgical bath and method involving the lance

ABSTRACT

A method for submerged injection of materials into a liquid pyrometallurgical bath by means of a lance, characterised in that a first gas consisting of or containing oxygen is conveyed to said bath along a first path within the lance, a combustible fluid is conveyed to said bath along another path within the lance, and a further gas consisting of or containing oxygen is conveyed to said bath along a further path within the lance, the first path being arranged so that the first gas acts as a coolant for the lance. A lance for submerged injection of materials into a liquid pyrometallurgical bath, comprising an outer end portion to be submersed in the bath, an outer lengthwise extending tubular member, an inner lengthwise extending tubular member positioned within the outer tubular member, an annular duct being thereby defined between the outer and inner tubular members for conveying a gas consisting of or containing oxygen to an open outer end thereof, a conduit positioned within and extending lengthwise of the inner tubular member for conveying further gas consisting of or containing oxygen to the outer end portion of the lance, a lengthwise passage being thereby defined between the inner tubular member and the conduit for conveying combustible fluid to the outer end portion of the lance, at least one port providing communication between the passage and the annular duct and at least one exit passageway providing communication between the conduit and the annular duct at a location downstream of the port or ports, for directing the further gas flowing from the conduit into the annular duct.

This invention relates to a lance for immersion in a pyrometallurgicalbath and a method involving the lance.

In carrying out a bath smelting operation, it is necessary to injectfuel with air or oxygen enriched air below the surface of the bath toachieve both heating of the bath and mixing by means of the turbulencecreated by the passage of gas bubbles through the bath. Injection of thegas and fuel may be achieved by three main methods, namely:

(1) using side blown tuyeres as in the Pierce-Smith converter or thezinc slag fuming furnace,

(2) bottom entry tuyeres, usually of the hydrocarbon shrouded Savard-Leeinjector type, or

(3) through top entry lances which must be cooled to prevent burningaway of the tip of the lance.

In the Mitsubishi process a steel lance is located at the surface of theslag bath and is allowed to burn away at a slow rate while it is fedinto the bath from above and rotated to ensure even wear.

The above described prior art processes are "submerged combustion"processes. As an alternative to submerged combustion, a water cooledlance may be located above the level of the bath, and air or oxygen(with or without fuel) may be blown at supersonic velocity into the bathas in the LD oxygen steel making process.

One form of submerged combustion lance is described in U.S. Pat. No.4,251,271. This employs cooling by means of the air used for combustionof the fuel. In this case the dimensions of the lance are arranged sothat the gas flow rate and the velocity of flow through the lance tubecause a layer of slag to solidify on the outer surface of the lance andprotect it from attack by the bath. In this type of lance a swirler isused to increase the gas velocity and enhance the heat transfer throughthe wall to the flowing gas. The swirler also serves the purpose ofimproving the mixing between the air and the fuel which is deliveredthrough a central pipe. While this type of lance has been usedsuccessfully in a number of bath smelting applications, it suffers froma number of disadvantages.

Thus, to achieve the required heat transfer near the tip of the lance,the gases are accelerated up to velocities approaching Mach 1. Whenattempts are made to force the air to flow at a higher rate the spiralpassages in the swirler behave as choked ducts. A very large increase inpressure is then necessary to compress the gas and achieve higher massflow rate. Flexibility and turndown with this lance is limited by thenecessity to maintain a minimum flow down the lance to ensure adequatecooling. Again, because the combustion air is the coolant for thislance, it is not possible to enrich this air with oxygen much above 35%oxygen, since with higher oxygen contents the tip of the lance may burnaway.

Broadly speaking, in the present invention these limitations are atleast lessened by using an annular duct through which cooling air flowsat sufficiently high mass flow rate and velocity to cool an outer lancetubular member.

According to one aspect of the present invention there is provided alance for submerged injection of materials into a liquidpyrometallurgical bath, comprising an outer end portion to be submersedin the bath, an outer lengthwise extending tubular member, an innerlengthwise extending tubular member positioned within the outer tubularmember, an annular duct being thereby defined between the outer andinner tubular members for conveying a gas containing oxygen to an openouter end thereof, a conduit positioned within and extending lengthwiseof the inner tubular member for conveying further gas consisting of orcontaining oxygen to the outer end portion of the lance, a lengthwisepassage being thereby defined between the inner tubular member and theconduit for conveying combustible fluid to the outer end portion of thelace, at least one port providing communication between the passage andthe annular duct and at least one exit passageway providingcommunication bet-ween the conduit and the outer end portion of theouter tubular member at a location downstream of the port or ports, fordirecting the further gas flowing from the conduit into the outer endportion of the lance.

According to another aspect of the invention, there is provided a methodfor submerged injection of materials into a liquid pyrometallurgicalbath by means of a lance, characterised in that a first gas orcontaining oxygen is conveyed to said bath along a first path within thelance, a combustible fluid is conveyed to said bath along another pathwithin the lance, and a further gas containing at least 35% oxygen isconveyed to said bath along a further path within the lance, the firstpath being arranged so that the first gas acts as a coolant for thelance.

According to a further aspect of the present invention there is provideda method for injecting materials into a liquid pyrometallurgical path,characterised in that the lance as described above is positioned so thatthe outer end portion of the lance is submersed in the bath and the gascontaining oxygen, the further gas consisting of or containing oxygenand the combustible fluid passed along the lance, through the annularduct, and through the conduit and the lengthwise passage, respectivelyto exit at the outer end portion of the lance.

The annular duct may be divided near the open outer end thereof to forma plurality of duct portions. The plurality of duct portions may beprovided by at least two radial baffles extending between the inner andouter tubular members. Preferably, the at least two radial baffles arein spiral form thereby to impart swirl to the gas flowing within theannular duct.

The term "combustible fluid" as used herein will be understood toinclude (but not be limited to) combustible gases, such a natural gas orother gaseous fuels; vaporising fuels, such as oils or liquefiedpetroleum gas; and particulate solid or liquid fuels, such as oil orpulverised coal entrained in a carrier gas.

Preferably, when carrying out the method of the invention, thecombustible fluid is passed through the lengthwise passage for exittherefrom via said port(s). The port or ports may be in the form of ahole or a slot, preferably located substantially within 1000 mm from theopen outer end of the annular duct. When more than one port is presentthese may be spaced around the circumference of the annular duct.

In one form of the lance the lengthwise passage may be terminated at itsouter end by a closure (through which the conduit passes) with thecombustible fluid passing through radial ports into the annular duct.Alternatively, the lengthwise passage may be a partial closure withaxial ports providing outflow of fluid directly into the outer portionof the outer tubular member.

The conduit may extend through the closure or partial closure so as toprovide outflow of further gas through its open end or alternativelythrough exit passageways. Preferably, the pen end of the conduit mayterminate at a location not more than substantially at a distance equalto one outer tubular member diameter upstream and three diametersdownstream of the open end of the outer tubular member, or alternativelyexit passageways may be located within a distance equal to threediameters past the end of the outer tubular member.

Typically, the gas employed in using the lance is air. The gas pressuremay be in the range 50 to 100 kPa. This may be supplied by a suitableblower while "turn up" is achieved by burning additional fuel with arelatively small volume of additional oxygen delivered through saidconduit near the open outer end of the annular duct. In anotherembodiment, liquid fuel may be delivered through the lengthwise passageand at least one port provided with an atomising nozzle.

By introducing some or all of the oxygen separately through the conduitit is possible to achieve higher levels of enrichment. The extent towhich enrichment is possible depends on the scale of the operation andthe application. However, it will be appreciated that small diameterlances (25 mm diameter) have been operated with effective oxygenenrichment levels of 70%.

Conveniently, the inner and outer tubular members are coaxial and theconduit may likewise be coaxial with the inner tubular member. The lancemay be composed of steel at the outer portion of the lance to besubmerged in the bath, preferably stainless steel. In use, a solidifiedslag layer forms on the lance. The dimension of the annular duct ispreferably such that the required cooling air flow rate can be achievedat low supply pressures, typically not exceeding 100 kPa as describedabove.

When the aforementioned additional fuel is required, in excess of thatwhich can be burned with the supplied quantity of air, the additionaloxygen is injected through the conduit into a stream of fuel and air ata location close to the axis of the lance and close to the open end ofthe lance so that it does not mix completely with the fuel/air mixturein the short time lapse before the mixture passes through the open endof the lance. Contact between strongly oxygen enriched air and the outertubular member can therefore be avoided, but the oxygen is available forcombustion in the flame immediately beyond the lance tip. Thus, thelowest heat input to the bath can be achieved by burning fuel in the airflowing from the annular duct at the minimum rate necessary to form theprotective slag layer. The described "turn-up" to higher heat input,achieved by burning additional fuel with oxygen, is effected withoutincreasing the flow of cooling air (and therefore the supply pressure).

According to a still further aspect of the present invention there isprovided a lance for immersion into a liquid pyrometallurgical bath,comprising an outer tubular member, an inner tubular member which isconcentric with the outer tubular member, a conduit being located withinthe inner tubular member, an annulus being defined between the outer andinner tubular members, said annulus being open at an outer end thereofand through which air flows at a sufficiently high flow rate andvelocity past an inner surface of the outer tubular member to cool theouter tubular member and to cause a protective layer of the liquid inthe bath to solidify on said outer tubular member, the annulus beingdivided near the open outer end into a plurality of ducts by means of atleast two radially extending baffles.

An embodiment of the invention will now be described by way of exampleonly with reference to the accompanying drawings in which:

FIGS. 1 and 2 are fragmentary cross-sectional views of a lance accordingto the present invention while FIGS. 1a, 1b and 2a show end views; and

FIGS. 3 and 4 are fragmentary views as in FIGS. 1 and 2, but showing amodified form of a lance according to the present invention.

A coaxial outer tubular member 1 and inner tubular member 2 form anannular duct 3 through which air for cooling and partial combustionflows. The flow is downwardly as shown in the drawings towards an outerend portion of the lance. In use, the outer end portion of the lance issubmerged in the bath.

At the outer end portion of the lance, the inner tubular member 2 issupported by baffles 5 from the outer tubular member 1. A conduit 7 ispositioned coaxially within inner tubular member 2 so as to define anlengthwise passage 12 between the inner surface of the inner tubularmember 2 and the outer surface of the conduit. The conduit is secured inposition by means of members 8 and 9 which provide attachment betweenthe inner tubular member and the conduit. The member 9 is located at thelower end portion of the inner tubular member 2.

The member 9 is of annular form, substantially closing or partiallyclosing the inner tubular member 2 at its lower end. The conduit 7extends coaxially through the member 9 so as to provide for outflow offurther gas through the end of the conduit 11 which is located a shortdistance below the lower surface of the member 9.

The baffles 5 may be of spiral form to impart swirl to air moving withinthe annular duct 3 or may be straight baffles which terminate in a shortspiral portion.

Ports 6, in the form of holes or slots extend through the side wall ofthe inner tubular member 2. Two alternative positions are indicated--6is at the lower end of 2 and immediately above member 9 while 6a allowsentry of fuel into the swirler region around tubular member 2. Byappropriate choice of the size of the holes and slots and the positionwithin the swirler region it is possible to regulate

a) the proportion of fuel entering the swirler region; and

b) the extent of mixing.

By such means, it is possible to regulate the intensity of combustion.In FIG. 1b parts 6b are also shown which extend axially through member 9in order to provide for outflow from the tubular member 2. Ports 6a and6b may be provided instead of or additionally to port 6. In the lance ofFIGS. 1a and 3, the ports 6b are not provided, only ports 6 and/or 6a.

Powdered coal transported by carrier gas flows down the lengthwisepassage 12 into oxygen containing gas which passes down the annular duct3. This inflow occurs via the ports 6, 6a and/or 6b. Oxygen is deliveredthrough the conduit 7 to emerge into the outer end of member 13 via theend of the conduit 11 as shown in FIGS. 1 and 2 and/or the exitpassageways 10 as shown in FIGS. 3 and 4 at locations downstream of thelocations at which the carrier gas and powdered coal emerge from theports 6, 6a or 6b. An atomizing nozzle 14 may also be provided in atleast one port so as to deliver fuel through the lengthwise passage 12.

The inner tubular member 2 may have an enlarged portion towards theouter end of the inner tubular member as indicated by broken lines 4 ofFIGS. 1 to 4.

To assist the flow from the lengthwise passage 12 into the annular duct3, the member 9 may have a frusto-conical upper surface 9a and the ports6 may be angled as viewed in cross-section so as to correspond with anangle of the frusto-conical upper surface of the member 9.

As shown in FIGS. 3 and 4, the member 9, shown in FIG. 1, may also havea further frusto conical surface portion at its lower end to form afurther member 13 which projects across the open outer end of theannular duct 3 at a location below an end of the outer tubular member 1.By this arrangement, lateral momentum is imparted to gases leaving thetip of the lance. The further member 13 may also have a frusto-conicalupper surface 13a. In FIGS. 3 and 4, the outer end of the conduit 7 isclosed by further member 13 and outflow from the conduit 7 occursthrough exit passageways 10 through further member 13. Provision couldalso be made for outflow from conduit 7 via the end of the conduit 11 asin the lance of FIGS. 1 and 2. Alternatively, the lance of FIGS. 1 and 2may he provided with an exit passageway 10 as shown in FIGS. 3 and 4.

The general operation of the lance as shown is as follows:

1. Combustible gas, or finely divided coal conveyed by carrier gas, ispassed through the lengthwise passage 12 within the inner tubular member2 and is delivered into the high velocity air stream flowing in theannular duct 3 (which duct is divided by baffles 5) through thecircumferential ports 6 or 6a at a location substantially within 1000 mmfrom the open outer end of the annular duct 3 or alternatively throughthe axial ports 6b.

2. Oxygen is conveyed through the conduit 7 in the inner tubular member2 and is injected through the exit passageways 10 (FIGS. 3 and 4) or 11(FIGS. 1 and 2) into the stream of air and fuel at a location preferablydownstream of the injection points of coal fuel.

3. The inner tubular member 2 forming the annular duct 3 preferablyterminates at a location which may vary between one meter inside theopen end of the outer tubular member and several outer tubular memberdiameters beyond the end of the outer tubular member.

4. The lance may be operated at an air pressure which may typically beas low as 50-100 kPa which can be supplied by a blower, while "turn-up"is achieved by burning additional fuel with a relatively small volume ofoxygen delivered near the outer end portion of the lance.

5. In another embodiment of the lance shown in FIGS. 2 and 4, liquidfuel may be delivered through atomising nozzles into the high velocityair stream.

As described above, the inner tubular member 2 forming the annular ductmay have an enlarged portion as identified by broken lines 4. Thisenlargement may be for distance of up to 2 meters from the outer end ofthe inner tubular member and may serve to decrease the annular area ofannular duct 3 and to impart high velocity to the gases flowing throughthe annular duct, the enlargement being such that at the highest airflow rate at which the lance is operated the velocity increases fromapproximately 100 meter/see in the wide annular section in the upperportion of the lance to approximately Mach 0.9 in the reduced annularduct at the open end of the outer tubular member 1.

Alternatively, all or part of the increase in velocity towards the openouter end of the annular duct may be achieved by shaping the radiallyextending baffles 5 into spirals also as discussed above for all or partof their length. This imparts swirl to the gases flowing from the lanceand therefore enhances mixing between the air, combustible fluid andoxygen. The swirl angle is preferably designed so that the helicalvelocity does not exceed Mach 0.9 and generally it is preferred that theswirl angle of the or each radial baffle is such that choked flow isavoided and low pressure operation is attained. However, in operation itis possible to raise the supply pressure to achieve choked flowoperation in which the helical velocity reaches Mach 1.

The main purpose of the increase in velocity towards the outer endportion of the lance is to achieve very high rates of heat transfer overthat section of the lance which is submerged in the bath to ensureadequate cooling to preserve the coating of solidified slag. The highexit velocity also helps to disperse the gases entering the bath. When aswirler is employed, the gases also acquire lateral momentum whichprevents excessive penetration of gas bubbles below the tip of thelance. In cases where no swirl is imparted to the gases, lateralmomentum may be imparted by flaring the lower end of the inner tubularmember at the open end of the annular duct 3. In cases where it isintended to use the lance in a strongly reduced slag bath, e.g., in zincslag fuming, the majority of the coal is used as fuel while theremainder (typically one-quarter to one-third of the total coal) servesas reductant in the bath. The fraction which is used as fuel must befinely divided typically 100% minus 75 micrometer, in order to achievegood combustion in the flame at the tip of the lance. The fraction whichis used as reductant may vary in size up to the largest size which maybe transported through the delivery tube into the gas stream.Alternatively, this fraction may be charged as lumps onto the surface ofthe bath. The coal fraction which is used as fuel should also have asufficiently high volatile content (typically greater than 10%) so thatit ignites rapidly in the burning zone.

Where the lance is intended for use in an oxidative smelting system suchas copper smelting, direct lead smelting or nickel smelting, therequirement for rapid combustion is not severe. The coal need only bereduced in size to the extent necessary to allow it to be transported tothe combustion zone of the lance. The reason for this is that a bathcontaining a matte and/or slag phase acts as a very efficient oxygencarrier, so that the bath may be over-oxidised by excess unreacted airat the tip of the lance and subsequently reduced by the injected coalparticles as they are mixed into the bath by the turbulence induced bythe injected gases.

Further embodiments of the invention will now be described withreference to the following Examples. These examples are not to beconstrued as limiting the invention in any way.

EXAMPLE 1

Smelting slag at 60 kg/h with gas as fuel at 60% O₂ at 1300°-1350° C.and also at 1400°-1450° C.

The furnace was preheated to 1250° C. then a lance was lowered into thefurnace. The lance comprised three concentric stainless steel tubes, a25.4 mm outside diameter tube of wall thickness of 1.6 mm, an inner fueltube 15.8 mm outside diameter and wall thickness 1.6 mm and a centraloxygen tube 6 mm outside diameter with a 0.8 mm wall. The upper end ofthe lance was fitted with connections which provided attachments forair, natural gas and oxygen supplies. At the lower end a double startswirler of 55 mm pitch was fitted over 150 mm of the fuel tube,terminating 50 mm from the end. The oxygen tube extended 10 mm past theend of the fuel tube which itself was within 30 mm of the end of thelance outer tube.

A molten slag bath was prepared by lowering the lance and melting slagin the vessel by impinging hot combustion gases from the lance on thesurface--slag was added until a sufficient depth of molten slag wasobtained to allow immersion of the lance into the bath. The lance waslowered until the tip was just above the slag surface and remained thereuntil a protective layer of slag coated the lance outer tube after whichperiod the lance was immersed into the molten slag.

Granulated slag was fed continuously to the furnace at 50 kg/h for 15minutes, the oxygen content was increased to 50% with an oxygen flowrate of 18 Nm³ /h and air flow rate of 31 Nm³ /h--the natural gas ratewas 13.1 Nm³ /h.

After this period the slag feed rate was increased to 60 kg/h andmaintained at that rate for 55 minutes.

The oxygen content was increased to 60% with air flow rate of 28 Nm³ /h,oxygen flow of 26 Nm³ /h and natural gas rate of 16.9 Nm³ /h. Thetemperature was maintained at 1300°-1350° C. by applying a heat load of51.8 Mjoules to the furnace.

After reaching furnace capacity the lance was raised and inspection ofthe lance tip showed minimal surface attack.

At this point approximately 60 kg of slag was tapped from the furnaceand smelting continued with 60 kg/h of slag with air flow rate of 28.2Nm³ /h, oxygen flow of 26 Nm³ /h and gas rate of 16.9 Nm³ /h--again aheat load of 34 Mjoules was applied to maintain the temperature at1400°-1450° C. for 1 hour after which the slag was poured from thefurnace into moulds. Inspection of the tip showed that it eroded only afew millimeters.

EXAMPLE 2

Testing lance materials--Type 304 S.Steel, 253 MA and Chromed steel inslag at 1300°-1400° C. at 60%, 65% and 70% oxygen enrichment

A 60 kg molten slag bath was prepared and the lance configuration anddimensions of Example 1 were employed.

In the first trial, a lance with an outer tube of type 304 stainlesssteel was splash coated in accordance with the method. The lance wasthen immersed into the bath and the oxygen enrichment set at 60% oxygen,the air flow set at 27.7 Nm³ /h, gas rate of 17.7 Nm³ /h and oxygen rateof 26.5 Nm³ /h and the temperature was maintained at 1300°°-1400° C. byimposing a heat load of 68 Mjoule on the furnace after 30 minutes theoxygen enrichment increased to 65% oxygen, the air flow maintained at27.7 Nm³ /h and increases in the gas to 21.0 Nm³ /h and oxygen 34.0 Nm³/h respectively, the temperature was maintained at 1300°-1400° C. byincreasing the heat load to 114 Mjoule. The lance was raised after 30minutes and inspection of the tip showed about 10 mm of the outer haderoded. The lance was again splash coated and then lowered into the slagand the oxygen enrichment increased to 70% oxygen, the air flowmaintained at 27.7 Nm³ /h and increases in the gas to 25.1 Nm³ /h andoxygen to 41.8 Nm³ /h respectively, the temperature was again maintainedat 1300°-1400° C. by increasing the heat load to 206 Mjoule. The lancewas raised after 30 minutes and inspection of the tip showed no furthererosion.

In the second trial, the lance outer tube was replaced with a type 304stainless steel which had been hard chrome plated. The lance was againsplash coated in accordance with the method and the tip immersed intothe slag bath. The lance was then immersed into the bath and the oxygenenrichment set at 60% oxygen, the air flow set at 27.7 Nm³ /h, gas rateof 17.7 Nm³ /h and oxygen rate of 26.5 Nm³ /h and the temperature wasmaintained at 1250°-1400° C. by imposing a heat load of 68 Mjoule on thefurnace after 30 minutes the oxygen enrichment increased to 65% oxygen,the air flow maintained at 27.7 Nm³ /h and increases in the flow ratesof gas to 21.0 Nm³ /h and oxygen to 34.0 Nm³ /h respectively, thetemperature was maintained at 1300°-1400° C. by increasing the heat loadto 114 Mjoule. The lance was raised after 30 minutes and inspection ofthe tip showed it had eroded back to an equilibrium distance from theoxygen tube (ie a distance of 10 mm). The lance was again splash coatedand then lowered into the slag and the oxygen enrichment increased to70% oxygen, the air flow maintained at 27.7 Nm³ /h and increases in thegas to 25.1 Nm³ /h and oxygen to 41.8 Nm³ /h respectively, thetemperature was again maintained at 1300°-1400° C. by increasing theheat load to 206 Mjoule. The lance was raised after 30 minutes andinspection of the tip showed no further erosion.

EXAMPLE 3

Copper smelting at 50 kg/h with natural gas fuel with 60% oxygenenrichment at 1300°-1400° C.

The lance configuration and dimensions of Example 1 were employed. Thefurnace was preheated to 1300° C. then a lance was lowered into thefurnace and a 40 kg molten slag bath was prepared as in Example 1.

The lance was then lowered until the tip was just above the slag surfacewhere it remained until a protective layer of slag coated the lanceouter tube, after which period the lance was immersed into the moltenslag.

Copper concentrate pellets were then fed continuously to the furnace at50 kg/h for 35 minutes with the oxygen enrichment controlled at 50% withan oxygen flow rate of 36 Nm³ /h and air flow rate of 37 Nm³ /h--thenatural gas rate was 15.9 Nm³ /h. After this period the enrichment wasincreased to 60% oxygen and maintained at that level for 2 hours. Theair flow rate was set at 37 Nm³ /h, oxygen flow of 36.1 Nm³ /h andnatural gas rate of 15.9 Nm³ /h. The temperature was maintained at1300°-1350° C. by applying a heat load of 147-188 Mjoules to thefurnace.

After reaching furnace capacity the lance was raised and the furnacecontents tapped into moulds. Inspection of the lance showed minimalsurface attack and about 3 mm erosion of the tip.

The same lance inner tubes were used for all the examples and the type304 stainless steel lance outer tube was also used in Example 1, thefirst trial in Example 2 and in this example.

In the third, trial the lance outer tube was replaced with a type 253MAsteel sheath with the tip set back 10 mm from the oxygen tube. The lancewas again splash coated in accordance with the method and the tipimmersed into the slag bath. The lance was then immersed into the bathand the oxygen enrichment set at 60% oxygen, the air flow set at 27.7Nm³ /h, gas rate of 17.7 Nm³ /h and oxygen rate of 26.5 Nm³ /h and thetemperature was maintained at 1300°-1400° C. by imposing a heat load of68 Mjoule on the furnace after 30 minutes the oxygen enrichmentincreased to 65% oxygen, the air flow maintained at 27.7 Nm³ /h andincreases in the gas to 21.0 Nm³ /h and oxygen 34.0 Nm³ /h respectively,the temperature was maintained at 1300°-1400° C. by increasing the heatload to 114 Mjoule. The lance was raised after 30 minutes and inspectionof the tip showed toughening of the tip but no significant erosion. Thelance was again splash coated and then lowered into the slag and theoxygen enrichment increased to 70% oxygen, the air flow maintained at27.7 Nm³ /h and increases in the gas to 25.1 Nm³ /h and oxygen to 41.8Nm³ /h respectively, the temperature was again maintained at 1300°-1400°C. by increasing the heat load to 206 Mjoule. The lance was raised after30 minutes and inspection of the tip showed no further erosion.

EXAMPLE 4

Smelting slag at 50 kg/h with pulverised coal as fuel with 60% O₂enrichment at 1300°-1350° C.

The lance configuration and dimensions of Example 1 were employed. Thefurnace was preheated to 1300° C. then a molten slag bath (40 kg) wasprepared by lowering the lance and melting slag in the vessel byimpinging the hot combustion gases from the lance on the topsurface--natural gas was used as fuel at a rate 13.1 Nm³ /h, air flowrate of 46 Nm³ /h and oxygen flow rate of 14.8 Nm³ /h.

Slag was added until a sufficient depth of molten slag was obtained toallow immersion of the lance into the bath.

The lance was splash coated according to the method then immersed intothe molten slag.

Granulated slag was fed continuously for 20 minutes to the furnace at 50kg/h, the oxygen enrichment was controlled at 50% with an oxygen flowrate of 22.5 Nm³ /h and air flow rate of 38.9 Nm³ /h and the pulverisedcoal fuel rate was 20 kg/h. The oxygen enrichment was increased to 60%for a further 80 minutes with an oxygen flow rate of 25.2 Nm³ /h, airflow rate of 26 Nm³ /h and pulverised coal rate of 20 kg/h and thetemperature was maintained at 1300°-1350° C. by imposing a heat load of147 Mjoule to the furnace. These conditions provided low pressure (50kPa), non-choked flow in the lance. To demonstrate the effect of chokedflow, the rates of oxygen, air and coal were then increased to 30.2 Nm³/h, 31.2 Nm³ /h and 24 kg/h, respectively. This led to choked flow whichrequired a significant increase in the air pressure, to 140 kPa, tomaintain the desired air flow.

After smelting slag for 2 hours the lance was lifted and the contents ofthe furnace poured into moulds. Inspection of the lance showed thatthere was no erosion of the tip of the lance.

We claim:
 1. A lance for submerged injection of materials into a liquidpyrometallurgical bath, comprising an outer lengthwise extending tubularmember with an outer end portion to be submersed in the bath, an innerlengthwise extending tubular member positioned within the outer tubularmember, an annular duct being thereby defined between the outer andinner tubular members for conveying a gas containing oxygen to an openouter end thereof, a conduit positioned within and extending lengthwiseof the inner tubular member for conveying further gas consisting of orcontaining oxygen to the outer end portion of the lance, a lengthwisepassage being thereby defined between the inner tubular member and theconduit for conveying combustible fluid to the outer end portion of thelance, at least one port providing communication between the passage andthe annular duct and at least one exit passageway providingcommunication between the conduit and the outer end portion of the outertubular member at a location downstream of the port or ports, fordirecting the further gas flowing from the conduit into the outer endportion of the lance.
 2. A lance as claimed in claim 1, characterised inthat the annular duct is divided near the open outer end to form aplurality of duct portions.
 3. A lance as claimed in claim 2,characterised in that the plurality of duct portions is provided by atleast two radial baffles extending between the inner and outer tubularmembers.
 4. A lance as claimed in claim 3, characterised in that the atleast two radial baffles are in spiral form thereby to impart swirl tothe gas flowing within the annular duct.
 5. A lance as claimed in claim4, characterised in that the swirl angle of each radial baffle is suchthat choked flow is avoided and low pressure operation is attained.
 6. Alance as claimed in claim 4, characterised in that the swirl angle ofeach radial baffle is such that the helical velocity does not exceedMach 0.9.
 7. A lance as claimed in claim 1 characterised in that theinner tubular member has an enlarged portion.
 8. A lance as claimed inclaim 7, characterised in that the enlarged portion is located towardsthe outer end of the inner tubular member.
 9. A lance as claimed inclaim 1, characterised in that the inner and outer tubular members arecoaxial.
 10. A lance as claimed in claim 1, characterised in that theconduit is coaxial with the inner tubular member.
 11. A lance as claimedin claim 1, characterised in that the dimension of the annular duct issuch that the desired gas flow rate can be achieved at a low supplypressure.
 12. A lance as claimed in claim 1, characterised in that anatomising nozzle is provided in at least one port so as to deliverliquid fuel through the lengthwise passage.
 13. A lance as claimed inclaim 1, characterised in that the lance is composed of steel.
 14. Alance as claimed in claim 1, characterised in that the outer end of theinner tubular member terminates at a location in the range from 1 minside the outer open end of the outer tubular member to a distancebeyond the end of the outer tubular member equal to twice the diameterof the outer tubular member.
 15. A lance as claimed in claim 1,characterised in that the lengthwise passage is terminated by a closureor a partial closure.
 16. A lance as claimed in claim 15, characterisedin that the closure has a frusto-conical upper surface to assist gasflow from the lengthwise passage into the annular duct.
 17. A lance asclaimed in claim 16, characterised in that the port or ports is/arelocated substantially adjacent the frusto-conical upper surface.
 18. Alance as claimed in claim 17, characterised in that the port or portsis/are angled so as to correspond with an angle of the frusto-conicalupper surface.
 19. A lance as claimed in claim 15, characterised in thatthe closure has a further frusto-conical surface portion at its lowerend which projects across the open outer end of the annular duct at alocation below the end of the outer tubular member.
 20. A lance asclaimed in claim 15, characterised in that the conduit extends throughthe closure so as to provide for outflow of gas through the open end ofthe conduit as well as or instead of through the at least one exitpassageway.
 21. A lance as claimed in claim 1, characterised in that theor each port comprises a hole or a slot.
 22. A lance as claimed in claim1, characterised in that there is more than one port and said ports arespaced around the circumference of the inner tubular member.
 23. A lanceas claimed in claim 1, characterised in that at least one port islocated substantially within 1000 mm open outer end of the annular duct.24. A lance as claimed in claim 1, characterised in that there is morethan one exit passageway and these are spaced around the circumferenceof the conduit.
 25. A lance as claimed in claim 1, characterised in thatthe exit passageway opens into the annular duct at a location not morethan substantially three times the inner diameter of the outer tubularmember upstream from the open outer end of the annular duct.
 26. A lanceas claimed in claim 1, characterised in that the outer end portion ofthe lance to be submerged in the comprises stainless steel.
 27. A lanceas claimed in claim 1 including at least one radial baffle extendingbetween the inner and outer tubular members, the or each baffle being inspiral form.
 28. A lance as claimed in claim 1, for immersion into aliquid pyrometallurgical bath, comprising an outer tubular member, aninner tubular member which is concentric with the outer tubular member,a conduit being located within the inner tubular member, an annulusbeing defined between the outer and inner tubular members, said annulusbeing open at an outer end thereof and through which air flows at asufficiently high flow rate and velocity past an inner surface of theouter tubular member to cool the outer tubular member and to cause aprotective layer of the liquid in the bath to solidify on said outertubular member, the annulus being divided near the open outer end into aplurality of ducts by means of at least two radially extending baffles.29. A method for submerged injection of materials into a liquidpyrometallurgical bath by means of a lance as claimed in claim 1,comprising the steps of: (a) conveying a first gas containing oxygen tosaid bath along a first path within the lance; (b) conveying acombustible fluid to said bath along another path within the lance; and(c) conveying a further gas containing at least 35% oxygen to said bathalong a further path within the lance, the first path being arranged sothat the first gas acts as a coolant for the lance.
 30. A method forinjecting materials into a liquid pyrometallurgical bath, comprising thesteps of:(a) positioning a lance comprising an outer lengthwiseextending tubular member with an outer end portion to be submerged inthe bath, an inner lengthwise extending tubular member positioned withinthe outer tubular member, an annular duct being thereby defined betweenthe outer and inner tubular members for conveying a gas containingoxygen to an open outer end thereof, a conduit positioned within andextending lengthwise of the inner tubular member for conveying furthergas consisting of or containing oxygen to the outer end portion of thelance, a lengthwise passage being thereby defined between the innertubular member and the conduit for conveying combustible fluid to theouter end portion of the lance, at least one port providingcommunication between the passage and the annular duct and at least oneexit passageway providing communication between the conduit and theouter end portion of the outer tubular member at a location downstreamof the port or ports, for directing the further gas flowing from theconduit into the outer end portion of the lance so that the outer endportion of the lance is submerged in the bath and passing the gascontaining oxygen, the further gas consisting of or containing oxygenand the combustible fluid along the lance, through the annular duct, theconduit and the lengthwise passage respectively to exit at the outer endportion of the lance.
 31. A method as claimed in claim 30 wherein thedimension of the annular duct is such that the desired gas flow rate canbe achieved at a low supply pressure.
 32. A method as claimed in claim31 wherein the supply pressure does not exceed 100 kPa.
 33. A method asclaimed in claim 31 wherein the supply pressure is raised to achievechoked operation in which the helical velocity reaches Mach
 1. 34. Amethod as claimed in claim 30 wherein oxygen is delivered through theconduit near the open outer end of the annular duct so as to achieve"turn-up".